8. Assessment of the Northern Rock Sole stock in the Bering Sea and Aleutian Islands Thomas Wilderbuer and Daniel Nichol November 2017 Executive Summary Northern rock sole (Lepidopsetta polyxystra) are assessed on a biennial stock assessment schedule as part of the National Marine Fisheries Service assessment prioritization plan implemented in 2017. For Bering Sea/Aleutian Islands partial assessments, an executive summary is presented to recommend harvest levels for the next two years. Please refer to last year’s full stock assessment report for further information regarding the stock assessment model (Wilderbuer and Nichol, 2016, available online at https://www.afsc.noaa.gov/REFM/Docs/2016/BSAIrocksole.pdf). A full stock assessment document with updated assessment and projection model results is scheduled to be presented in next year’s SAFE report. A statistical age-structured model is used as the primary assessment tool for the Bering Sea/Aleutian Islands northern rock sole assessment, a Tier 1 stock. This assessment consists of a population model, which uses survey and fishery data to generate a historical time series of population estimates, and a projection model, which uses results from the population model to predict future population estimates and recommended harvest levels. The data sets used in this assessment include total catch biomass, fishery age compositions, trawl survey abundance estimates and trawl survey age compositions. In a partial assessment year, the full assessment model is not rerun but instead a Tier 1 projection model with an assumed future catch is run to estimate the stock level in the next two years. This incorporates the most current catch information without re-estimating model parameters and biological reference points. A Tier 1 partial projection rule is implemented to estimate the 2019 ABC and OFL. The Tier 1 projection operates within the full assessment model by projecting estimates of the female spawning biomass, age 6+ total biomass, ABC and OFL ahead two years. Since the full assessment model is not rerun in this assessment, only the projected values from the 2016 assessment are available (2017 and 2018) whereby values for 2019 are not estimated. The 2019 values are determined by a linear fit to the 2017 and 2018 estimates. If the trend is increasing, then the 2019 values are a roll-over of 2018. If the trend is decreasing, the projected proportional decrease from 2017 to 2018 is applied to 2018 to get 2019. Summary of Changes in Assessment Inputs Changes in the input data: There were no changes made to the assessment model inputs since this was not a full assessment year. New data added to the Tier 3 projection model, used to forecast stock condition out to year 2030, included an updated 2016 catch estimate (45,006 t) and new catch estimates for 2017. The 2017 catch was estimated by setting the catch as of October 21, 2017 as the final 2017 catch (35,069 t). To estimate future catches through 2030, the catches that corresponded to the average F of the most recent 5 years were used. Changes in the assessment methodology: There were no changes in assessment methodology since this was an off-cycle year. Summary of Results For the 2018 fishery, the recommend harvest is the maximum allowable ABC of 143,100 t from the Tier 1 projection model. This ABC is 14% less than year’s ABC of 155,100 t. Reference values for BSAI RE/BS rockfish are summarized in the following table, with the recommended ABC and OFL values for 2018 in bold. As estimated or As estimated or specified last year for: recommended this year for: Quantity 2017 2018 2018 2019 M (natural mortality rate) 0.15 0.15 0.15 0.15 Tier 1a 1a 1a 1a Projected total (age 6+) 1,000,600 923,200 923,200 852,000 Female spawning biomass (t) 539,500 472,200 472,200 413,300 Projected B0 678,310 678,310 BMSY 257,000 257,000 257,000 257,000 FOFL 0.160 0.160 0.160 0.160 maxFABC 0.155 0.155 0.155 0.155 FABC 0.155 0.155 0.155 0.155 OFL (t) 159,700 147,300 147,300 136,000 maxABC (t) 155,100 143,100 143,100 132,000 ABC (t) 155,100 143,100 143,100 132,000 As determined last year for: As determined this year Status 2015 2016 2016 2017 Overfishing No n/a No n/a Overfished n/a No n/a No Approaching overfished n/a No n/a No The stock is not being subject to overfishing, is not currently overfished, nor is it approaching a condition of being overfished. The tests for evaluating these three statements on status determination require examining the official total catch from the most recent complete year and the current model projections of spawning biomass relative to BMSY% for 2017 and 2018. The estimated total catch for 2017 is 35,069 t, far below the 2017 OFL of 159,700 t; therefore, the stock is not being subjected to overfishing. The estimates of spawning biomass for 2017 and 2018 from the 2016 stock assessment are 539,500 t and 472,200 t, respectively. Both estimates are well above the estimate of BMSY% at 257,000 t and, therefore, the stock is not currently overfished nor approaching an overfished condition. Fishery Trends catch 100 90 80 70 60 50 40 catch (1,000s t) 30 20 10 0 1975 1980 1985 1990 1995 2000 2005 2010 2015 2020 year The northern rock sole catch in 2017 of 35,069 t is below the 1975-2017 long term average of 40,000 t, and well below the annual ABC in every year. Catches primarily are made during a late-winter/early spring roe fishery and also as bycatch in the yellowfin sole fishery. Retention rates are high, estimated at 98% in 2015. 400,000 BSAI northern rock sole 350,000 300,000 250,000 TAC 200,000 ABC 150,000 catch 100,000 50,000 0 Survey Trends The 2017 shelf trawl survey abundance estimate decreased about 11% from the 2016 estimate and has been in a downward trend since about 2008, currently about half of the peak value estimated for 1994. survey biomass 3500 3000 2500 2000 1500 1000 survey biomass (1,000s t) 500 0 1980 1985 1990 1995 2000 2005 2010 2015 year Projected female spawning biomass when fishing at 5 800000 year average F 700000 600000 500000 400000 300000 200000 100000 0 female spawning biomass (1,000s t) biomass (1,000s spawningfemale 2017 2019 2021 2023 2025 2027 2029 Bmsy LowCI_SSB Mean_SSB UpperCI_SSB The northern rock sole stock is projected to remain above the BMSY level of female spawning biomass while declining through 2024. catch/biomass 0.1 0.09 0.08 0.07 0.06 0.05 0.04 0.03 catch/total 6+biomass 0.02 0.01 0 1975 1980 1985 1990 1995 2000 2005 2010 2015 Appendix Estimating Northern Rock Sole recruitment in the last (most recent) 6 years of the assessment using environmental covariates Dan Cooper, Lauren Rogers and Tom Wilderbuer Difficulties exist in estimating northern rock sole recruitment at young ages since they do not appear in BSAI survey catches until age 3 and not in survey age sampling until age 4 or 5. They are estimated to be 25 and 40% selected by the survey trawl (males and females respectively) at age 3 and 95 and 98% selected at age 5. The age 4 and 5 fish that do end up in the age samples are quite rare, typically only 7 fish out of 500 on an annual basis. Therefore, there is not a lot of information to inform the stock assessment model estimates of year class strength for the last (most recent) 6 years. Some assessments provide estimates for the last 3 years by using an average of the estimated values to provide more credible values of year class strength. Here we propose to use two environmental covariates in regression modeling to estimate the unknown recruitment, and then compare those estimates with future estimates derived from fitting full age composition data in the stock assessment model. Studies on the influence of environmental variables on BSAI northern rock sole recruitment have shown that both on-shelf springtime winds (Wilderbuer et al. 2002, Wilderbuer et al. 2013) and above average water-temperatures in nursery areas (Cooper et al. 2014, Cooper and Nichol 2016) are positively correlated with northern rock sole recruitment. Spring wind direction was obtained from the Ocean Surface Current Simulation Model (OSCURS) and was classified as either on- or across-shelf or off-shelf, depending on the ending longitude position after 90 days of drift starting from a locale in a known spawning area. Water temperature effects were calculated from the percent of the known northern rock sole nursery area (Cooper et al. 2014) that is in the cold pool each year from annual trawl survey bottom temperature data. For most models, percentage of the northern nursery area covered by the cold pool was used as a continuous variable. In one model, the percent cold pool was used a categorical variable, dividing years into cold and not-cold categories under the hypothesis that there is some amount of cold pool coverage of the northern nursery area that inhibits use of the northern nursery area and precluded high overall recruitment for the EBS in that year. Both indices extend back to 1982 for this analysis. Estimates of female spawning stock biomass were also included in the analysis for model runs when recruitment was estimated from a Ricker stock-recruitment model with environmental variables.